photovoltaics in parallel

As with any voltage sources in parallel you will have a condition where their outputs are not the same. Currents will circulate in their internal resistances and may harm them or at least produce poor charging results. You could isolate the panels with diodes and lose a diode drop in voltage which in this case probably won't matter much.

Tom

That's what I was afraid of, to tell the truth. So just one diode on one positive lead? or both? Now that I think about it, I think I will need it on both 'cause I don't know which voltage will be higher. I'm going to go scribble it out...

rK

That shouldn't be necessary as photovoltaics are already diodes. They should current-share just fine.

Why not get a coulple of (milli)ammeters and check?

Cheers! Rich

Good advice but not all panels are constructed the same. Some will even _draw_ current at night. It is very typical to iolate the panels with Schottky Diodes. His may not need anything, a simple test as you suggest will determine.

Tom

They are diodes, but the orientaton is the reverse of what your isolation diodes might be. The open circuit voltage of a pv panel is its Vf, basically.

This causes a problem if panels are parallelled and one is in shade, or even if you have a single large panel in partial shade: some of the power collected by one panel ends up getting consumed by the other. I assume it acts like a big, diffuse infrared LED.

My gut feeling is that the OP's plan of adding another panel would work fine, as long as the two panels have the same number of junctions/ open circuit voltage and are both in about the same amount of light.

But as the sun moves across the garden, there will be a time that one is in complete shade and one is in complete sunlight.

The usual approach is to put the cells in series, and let the load (the battery under charge) draw the series down in voltage. This is because a common failure mode of solar batteries is for an individual diode to short out, and that doesn't hurt a series connection.

You may find that parallel operation doesn't charge as well (and that it needs more light to start charging).

Don't worry about overcharging, or making too high a voltage; the battery will regulate that.

Check this out -

I haven't read it in detail but I suspect it does not say exactly how it solves problems of shading etc, as it seems aimed at OEM designers who would talk directly to them. However it will help you understand what the issues are, and may give you some ideas on how to approach the problems.